Lyme disease was first recognized in the United States in 1975, after a mysterious outbreak of arthritis near Lyme, Conn. Since then, reports of Lyme disease have increased dramatically, and the disease has become an important public health problem in some areas of the United States. Lyme disease is an infection caused by Borrelia burgdorferi, a member of the family of spirochetes, or corkscrew-shaped bacteria.
Lyme disease is spread by the bite of ticks of the genus Ixodes that are infected with Borrelia burgdorferi. The deer (or bear) tick, Ixodes scapularis, which normally feeds on the white-footed mouse, the white-tailed deer, other mammals, and birds, is responsible for transmitting Lyme disease bacteria to humans in the northeastern and north-central United States. In these regions, this tick is also responsible for the spreading of babesiosis, a disease caused by a malaria-like parasite. On the Pacific Coast, the bacteria are transmitted to humans by the western black-legged tick, I. pacificus. Another newly recognized and serious disease that is transmitted by both I. scapularis and I. pacificus is human granulocytic ehrlichiosis, the pathogen of which is a rickettsial bacterium.
Ixodes ticks are much smaller than common dog and cattle ticks. In their larval and nymphal stages, they are no bigger than a pinhead. Adult ticks are slightly larger. Ticks can attach to any part of the human body but often attach to the more hidden and hairy areas such as the groin, armpits, and scalp. Research in the eastern United States has indicated that, for the most part, ticks transmit Lyme disease to humans during the nymphal stage, probably because nymphs are more likely to feed on a person and are rarely noticed because of their small size (less than two mm). Thus, the nymphs typically have ample time to feed and transmit the infection since ticks are most likely to transmit infection after approximately two or more days of feeding.
Tick larvae are smaller than the nymphs, but they rarely carry the infection at the time of feeding and are probably not important in the transmission of Lyme disease to humans.
Adult ticks can transmit the disease, but since they are larger and more likely to be removed from a person's body within a few hours, they are less likely than the nymphs to have sufficient time to transmit the infection. Moreover, adult Ixodes ticks are most active during the cooler months of the year, when outdoor activity is limited. Adults quest for hosts on grasses, shrubs and brush at heights of up to one meter. Immature Ixodes search for host animals from the tips of grasses and shrubs (not from trees) and leaf litter near the ground and transfer to animals or persons that brush against these substrates. Ticks only crawl; they do not fly or jump. Ticks found on the scalp usually have crawled there from lower parts of the body. Ticks feed on blood by inserting their mouth parts (not their whole bodies) into the skin of a host animal. They are slow feeders: a complete blood meal can take several days. As they feed, their bodies slowly enlarge.
Although in theory Lyme disease could spread through blood transfusions or other contact with infected blood or urine, no such transmission has been documented. There is no evidence that a person can get Lyme disease from the air, food or water, from sexual contact, or directly from wild or domestic animals. There is no convincing evidence that Lyme disease can be transmitted by insects such as mosquitoes, flies, or fleas. Campers, hikers, outdoor workers, and others who frequent wooded, brushy, and grassy places are commonly exposed to ticks, and this may be important in the transmission of Lyme disease in some areas. Because new homes are often built in wooded areas, transmission of Lyme disease near homes has become an important problem in some areas of the United States. The risk of exposure to ticks is greatest in the woods and garden fringe areas of properties, but ticks may also be carried by animals into lawns and gardens.
Geographic distribution of Lyme disease is wide in northern temperate regions of the world. In the United States, the highest incidence occurs in the Northeast, from Massachusetts to Maryland. Incidence is also notable in the North-central states, especially Wisconsin and Minnesota, and the West Coast, particularly northern California. For Lyme disease to exist in an area, at least three closely interrelated elements must be present in nature: the Lyme disease bacteria, ticks that can transmit them, and mammals (such as mice and deer) to provide food for the ticks in their various life stages. Ticks that transmit Lyme disease can be found in temperate regions that may have periods of very low or high temperature and a constant high relative humidity at ground level. Knowing the complex life cycle of the ticks that transmit Lyme disease is important in understanding the risk of acquiring the disease and in finding ways to prevent it: The life cycle of these ticks requires two years to complete. Adult ticks feed and mate on large animals, especially deer, in the fall and early spring. Female ticks then drop off these animals to lay eggs on the ground. By summer, eggs hatch into larvae. Larvae feed on mice and other small mammals and birds in the summer and early fall and then are inactive until the next spring when they molt into nymphs. Nymphs feed on small rodents and other small mammals and birds in the late spring and summer and molt into adults in the fall, completing the 2-year life cycle. Larvae and nymphs typically become infected with Lyme disease bacteria when they feed on infected small animals, particularly the white-footed mouse. The bacteria remain in the tick as it changes from larva to nymph or from nymph to adult. Infected nymphs and adult ticks then bite and transmit Lyme disease bacteria to other small rodents, other animals, and humans, all in the course of their normal feeding behavior. Lyme disease in domestic animals Domestic animals may become infected with Lyme disease bacteria and some of these (dogs, for instance) may develop arthritis. Domestic animals can carry infected ticks into areas where humans live, but whether pet owners are more likely than others to get Lyme disease is unknown.
There are proposed solutions to the prevention of transmission of tick-borne parasites to humans. For example, U.S. Pat. Nos. 5,648,398, 5,346,922, and 5,227,406 describe insect repellent compositions which are claimed to repel ticks. However, the use a repellent does not eliminate the vector itself but serves as a “chemical shield” against the ticks so that they will have to find another mammalian host. There are generally no known solutions to arrest the spread of Lyme disease and/or other diseases spread by ticks.